Problem installing gnome-doc-utils

[markd at macports.org]

>Half an hour normally. Then use `sudo port sync` to update your ports
>tree and to get the newest updates. Then run the command.

Are there good reasons not to do a selfupdate?  I haven't even documented
'port sync' in the new guide because I'm still waiting for an argument for
its utility.  Can someone tell me why we tell people to do a 'port sync'? 
I don't understood that.

Mark